1. Field of the Invention
The invention relates to a turbine blade of a gas turbine according to the preamble of claim 1.
2. Discussion of Background
The increase in output and the improvement in the efficiency of modern gas-turbine plants is achieved not least by an increase in the temperatures. However, since the thermal strength of the material of the gas turbine is limited, the components exposed to the maximum temperatures must be cooled. This also concerns in particular the guide and moving blades of the gas turbine.
To this end, the turbine blades are designed to be at least partly hollow in their interior and have one or more cooling passages. A cooling fluid flows through the latter, the cooling action resulting from convective heat transfer in the interior of the blade body. Additional film cooling is possible by portions of the cooling fluid being directed through openings in the blade body onto the outside of the turbine blade. A cooling-fluid film forms there and this screens the outside of the turbine blade from the hot working medium of the turbine (see DE 36 42 789 C2). Air which originates from the compressor of the gas-turbine plant or from an external source and is under positive pressure or even appropriately treated steam is known as cooling fluid.
Steam-cooling systems which first of all hold the steam in a closed cooling circuit, the steam originating from a steam circuit, vary from the technical point of view. The steam, which is heated by the convective cooling process, is fed again to the steam circuit (see EP 06 98 723 A2). Also known are open steam-cooling systems, in which the heated steam is directed via openings in the blade body onto the outside of the turbine blade. In addition, there are so-called hybrid steam-cooling systems having a closed main part and a cooling system which is open in the region of the trailing blade edge, the open cooling system being operated with steam or with air.
Compared with open steam-cooling systems and even compared with the known hybrid steam-cooling systems, closed steam-cooling systems have advantages relating to the process. The range of use of such systems is nowadays expanding, in particular on account of their higher efficiency. However, a closed steam-cooling system can be severely damaged by the penetration of foreign bodies into the cooling passage adjacent to the leading blade edge. Depending on the number and size of the holes forming in the leading blade edge during the impingement of the foreign bodies, so much cooling steam may escape that adequate blade cooling no longer takes place downstream of the point of penetration. As a result, the material overheats, for which reason serious consequential damage may occur.